The present proposal is a revised renewal of our RO1 grant investigating itch mechanisms, and is highly appropriate for the funding opportunity, PA-12-131, to improve translational and basic research to control itch in humans. Skin disease affects upwards of one-third of the US population at any given time and imposes a huge economic burden. Chronic itch frequently accompanies skin and systemic diseases, and is associated with scratching which can cause a vicious itch-scratch cycle that reduces the quality of life. Most types of chronic itch are poorly treated, establishing a compelling need to address the basis of chronic itch in order to develop more effective mechanisms-based treatments for this major medical and socioeconomic problem. During the funding period we developed models of itch using scratching behavior as a readout to investigate underlying neural mechanisms. The present proposal builds on these results, focusing on the poorly understood issue of descending modulation of itch. Specific Aim 1 uses an optogenetic approach to selectively transduce channelrhodopsin in noradrenergic neurons in the locus coeruleus/subcoeruleus of dbh-cre mice, and serotonergic neurons in the rostral ventromedial medulla of fev-cre mice. We will investigate the effects of optic activation of these brain areas on nocifensive (thermal paw withdrawal) and pruritogen-evoked scratching behaviors. We hypothesize that pain and itch behaviors are under opposing descending modulatory effects. Specific Aims 2 and 3 will use complementary electrophysiological methods to investigate brainstem mechanisms of descending modulation of spinal itch transmission. We will investigate how removal of descending influences from the brain affects the activity of spinal itch-signaling neurons. We will also investigate how conditions of acute itch or pain affect ON- and OFF-neurons in the rostral ventromedial medulla (RVM) that are thought to respectively facilitate or inhibit spinal nociceptive transmission (Specific Aim 2). We will also investigate spinal and supraspinal mechanisms by which scratching the skin inhibits itch (Specific Aim 3). These studies are expected to provide novel information regarding how descending pathways from the brain can modulate itch transmission. The results of this project have important translational significance for the development of new, mechanisms-based treatments for itch by enhancing inhibition and reducing facilitation of itch transmission.